Compounds capable of generating a free radical through decomposition upon exposure to light (hereinafter referred to as "free radical-generating agents") are well known in the field of graphic arts. They have been widely used as photopolymerization initiators in photopolymerizable compositions, photoactivating agents in compositions for free radical photography, and photo initiators for reaction systems that are catalyzed by acids generated by light. By using these free radical-generating agents, various light-sensitive materials can be produced that are useful in printing, duplication, copying and other image formation systems.
Organic halogen compounds are capable of forming a halogen free radical, such as a chlorine free radical and a bromine free radical, upon photolysis. The thus formed free radicals are satisfactory agents for drawing hydrogen and form acids in the presence of hydrogen donors. Applications of these organic halogen compounds to photopolymerization system of free radical photography are described in J. Kosar, Light Sensitive Systems, pp. 180-181 and 361-170, J. Wiley & Sons (New York, 1965).
Known compounds that generate halogen free radicals by the action of light and have widely been employed typically include carbon tetrachloride, iodoform, tribromoacetophenone and the like. However, these free radical-generating agents have a disadvantage in that they decompose with light of a considerably limited wavelength region. In other words, these compounds respond only to the ultraviolet region having shorter wavelengths than main wavelengths of commonly employed light sources. Therefore, they have poor capability of forming free radicals because of their inability to effectively utilize light in the range of from near ultraviolet to visible light.
In order to overcome the above-described disadvantage, it has been proposed to broaden the sensitive wavelength region by incorporating certain types of sensitizers, such as merocyanine dyes, styryl bases, and cyanine bases, as disclosed in U.S. Pat. Nos. 3,106,466 and 3,121,633. Although the incorporation of these sensitizers certainly succeeded in broadening the sensitive wavelength region of carbon tetrachloride or iodoform, the results were still unsatisfactory. This is because it was difficult to select such a sensitizer that exhibits good compatibility with the free radical-generating agent or other elements of a light-sensitive composition and also has high sensitivity.
Halogen free radical-generating agents sensitive to the near ultraviolet to visible light have been proposed in order to overcome the above-described problem. For example, U.S. Pat. Nos. 3,954,475, 3,987,037, and 4,189,323 disclose halomethyl-s-triazine compounds. These compounds respond to the near ultraviolet to the visible light region. However, their sensitivity for photolysis is relatively low since the irradiated light cannot be effectively absorbed.
Accordingly, there is a continuing problem that free radical-generating agents having senstivity to the near ultraviolet region to the visible region are still too low in photolysis sensitivity, and thus the sensitivity of the light-sensitive compositions containing them have not been as high as desired.